This invention relates to a method for regulating concentration and distribution of oxygen in Czochralski drawn silicon crystal rods through a combination of variation of a cusp magnetic field and variation of the magnitude of crystal and crucible rotation rates.
When a crystal is grown from a molten liquid stored in a container, the constituent materials of the container are partially dissolved in the molten liquid thereby migrating into the product crystal as impurities. At the melt temperature of silicon (about 1420.degree. C.), the surface of the silica (SiO.sub.2) crucible which is in contact with the melt dissolves. Some part of the dissolved silica evaporates from the surface of the melt as SiO (silicon monoxide). Another part of dissolved silica is incorporated into the growing crystal. The remainder of the dissolved silica is retained in the molten silicon. Thus, the silica crucible which is used to contain the silicon melt is the source of oxygen which is found in silicon crystals grown by the conventional Czochralski technique.
Oxygen in the silicon crystal may have both favorable and unfavorable effects. In the various heat treatment processes during the manufacture of various electrical devices, the oxygen in the crystal may cause crystal defects such as precipitates, dislocation loops and stacking faults or it may cause electrically active defects resulting in devices with inferior performance characteristics. The solid solution of oxygen in the crystal, however, increases the mechanical strength of silicon wafers and the crystal defects may improve the yield of conforming Products by entrapping contaminants of heavy metals. Accordingly, oxygen content of the silicon crystal is an important factor for product quality which must be carefully controlled in accordance with the requirements for the ultimate application for the silicon wafers.
The oxygen concentration in silicon crystals grown under Czochralski conditions prevalent in the industry in the early 1980s varied along the length of the crystal, for example, being higher at the seed end than in the middle and/or bottom or tang end of the crystal. In addition, there was variation in oxygen concentration along the radius of a cross-sectional slice of the crystal.
Frederick et al. proposed in U.S. Pat. No. 4,436,577 one method for regulating oxygen content and distribution of the oxygen content in silicon crystal rod drawn from the action of a seed crystal on a melt of silicon contained in a silica crucible. According to this method, the distribution of oxygen is controlled by rotating the crystal seed rod as drawn from the melt at a greater rate of rotation and in opposite direction to the rotation of the melt crucible rotation while increasing the crucible rotation rate as the crucible melt level diminishes.
Advances in silicon semiconductor technology in more recent years, however, have resulted in increased diameters of silicon crystals relative to that disclosed in the Frederick et al. patent. Consequently, increased molten charge weights and larger crucible diameters are required. In addition, semiconductor fabrication technology has evolved to require generally lower and more precisely controlled levels of oxygen in silicon wafers cut from the ingots. Accordingly, it has become increasingly difficult to homogenize oxygen content in all desired concentration ranges due to physical constraints imposed by the larger physical parameters which restricts the range of crystal and crucible rotation rates over which stable crystal growth is possible.
As a solution to this increasingly difficult oxygen control problem, attention has been given in recent years to the use of an axially-symmetrical and radial cusped magnetic field This method was suggested in Japanese Kokai Patent No. Sho 58[1983]-217493. According to this method, a pair of coils through which circular currents are permeated in opposing directions are configured above and below the molten liquid. As a result, a radial horizontal magnetic field is formed at the 1/2 position along the depth of the molten liquid. According to the applicant, the radial cusped magnetic: field restrains the flow of the molten liquid, thus stabilizing the melt and preventing contamination from the crucible.
Barraclough et al. have suggested an improvement to the cusp magnetic field method in WO 89/08731 (21.09.89). According to Barraclough et al., the magnetic field should have a component of field parallel to the axis of crystal rotation that is less than 500 gauss at the interface between the growing crystal and melt, with a value above 500 gauss at other parts of the melt and this distribution of magnetic field is maintained during the growth of the crystal.
Hirata et al. have suggested a different improvement in Japanese Kokai Patent No. Hei 1[1989]-282185. According to Hirata et al., migrating impurities such as oxygen are controlled by imposing a cusp magnetic field upon the melt, rotating the crucible and crystal in opposite directions, and rotating the crucible at a rotational speed greater than that of the crystal.
Hirata et al. have suggested yet another improvement in Japanese Kokai Patent No. Hei 2[1990]-55284. According to Hirata et al., migrating impurities such as oxygen are controlled by imposing a cusp magnetic field upon the melt and varying the ratio of the intensity of the magnetic field component which perpendicularly intersects the surface of the molten liquid and the intensity of the magnetic field component which perpendicularly intersects the bottom surface of the molten liquid. This ratio can be varied by (1) moving the coils relative to the crucible (while maintaining the distance between the coils constant), (2) changing the ampere turn ratio between the coils, or (3) changing the distance between the coils.
None of the cusp magnetic field methods proposed to date, however, has been entirely satisfactory. Under certain conditions, crystals grown in cusp magnetic fields exhibit poor axial and radial oxygen uniformity similar to that observed with axial fields. The problem, when experienced, tends to occur in the latter stages of the solidification process possibly as the result of an accumulation of oxygen or oxygen-containing compounds in a stagnation zone near the relatively strong vertically directed magnetic field in that area.